SUMMARY/ ABSTRACT Despite the great advancements in breast cancer research and treatment, approximately 42,000 US women are expected to die from breast cancer in 2019. Especially deadly are basal, triple- negative (TN) breast cancers. Basal TNBC are more prevalent in African American and Hispanic women and there are no biological therapies available to treat them. The E2 Promoter Binding Factors (E2Fs) are a group of eight transcription factors under retinoblastoma (Rb) control that regulate several cellular processes including DNA replication, genomic stability and apoptosis. Broadly, E2Fs can be divided into transcriptional activators (those that promote gene transcription) and repressors (those that halt gene transcription). Preliminary data I present here shows 67% basal/TN breast cancers overexpress the E2F activators. It is known that the Rb pathway is involved in the epithelial-to-mesenchymal transition (EMT), an early process that lead to metastasis; yet the involvement of the E2F activators driving EMT in breast cancer is still under investigation. Our published data demonstrated that overexpression of E2F activators (E2F1-3) leads to increased expression of mitotic regulators (MPS1, NEK2, SGO1, among others). Furthermore, we recently published that that MPS1 promotes EMT and invasion and NEK2 (an E2F target) generates pre-invasive protrusions in 3D culture models. Hence, we question to what extent deregulation of E2Fs activators promotes EMT, invasion, and metastasis through SGO1. Specifically, we will focus our research in E2F3 because it is overexpressed in 42% basal breast cancers and it we already showed that E2Fs regulate the stability of SGO1 protein levels. Our central hypothesis is that the overexpression of E2F3 contributes to tumorigenesis by promoting EMT through the induction of the transcription of SGO1, thus leading to increased invasion and metastasis. To test our central hypothesis, we will develop the following two specific aims: (1) Determine the effects of E2F3 and SGO1 in EMT, invasion, and migration in metastatic, triple- negative breast cancer cells. (2) Determine the effects of E2F3 knockout in MDA-MB-231 cell lines using CRISPR/Cas9 genome editing in NOD scid gamma mouse models. We expect to identify a mechanism of how E2F3 overexpression promotes EMT through SGO1 in breast cancer, thus contributing to migration, invasion, and metastasis. If the expected outcomes are met, small molecules may be designed to improve current therapies to target downstream effectors of E2F3, since E2Fs cannot be targeted.